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load('umatgen.py')
NH=umat(Implicit=True, project='Tets_NH',
outfile="NH", outroutine="NH", FEAFormat='LSDyna')
C=NH.C
parG, parLambda=var('G, Lambda')
# calculate the isochoric GST invariant
NH.W=(parG/2*(I1(C)-3-2*log(NH.Jacobian))
+parLambda/2*(log((NH.Jacobian)))^2)
show(NH.W)
NH.write()